It is widely known that when an electric motor is inverter-driven, a surge voltage due to high-speed switching in an inverter is generated to affect insulation of a motor winding. Such a surge voltage is called “inverter surge” and may sometimes reach double or more the rated voltage. When the inverter surge is applied to the motor winding, a partial discharge may occur inside or outside the winding. Such a partial discharge may cause degradation of coating of an enamel wire constituting the winding. Degradation of the coating may lead to insulation breakdown. Thus, there is desired an insulation design of the electric motor that prevents the partial discharge from occurring even when the inverter surge is applied to the winding.
Conventionally, insulation performance of the motor winding is evaluated based on partial discharge characteristics obtained by application of an ordinary alternate voltage, especially, a partial discharge starting voltage. However, a potential distribution in the winding differs between when the ordinary alternate voltage is applied and when the surge voltage is applied. Therefore, the insulation performance of the inverter-driven electric motor is desirably evaluated by application of an impulse voltage simulating the surge voltage. Further, in partial discharge measurement conducted by application of the impulse voltage, voltage changes steeply at rising time (at application start time) of the impulse voltage, so that there is required a different approach from the partial discharge measurement conducted by application of the ordinary alternate voltage.
Non-Patent Document 1 describes a guideline of a partial discharge measurement method that repeatedly applies the impulse voltage in one period. More specifically, Non-Patent Document 1 defines that an impulse voltage applied when a partial discharge frequency, which is the number of the partial discharges, with respect to the number of all the applied impulse voltages (hereinafter, referred to as “number of impulses”) reaches a specified frequency or more is regarded as a partial discharge starting voltage, which is called a partial discharge starting voltage in repeated impulse voltage.
Further, Non-Patent Documents 2 and 3 describe a partial discharge measurement method (all-period partial discharge measurement method) that repeatedly applies the impulse voltage for all periods. The following more specifically describes the all-period partial discharge measurement method.
FIG. 12 is a timing chart of the all-period partial discharge measurement method, illustrating an impulse voltage to be applied for each period (data segment in the drawing) to a twist pair sample simulating the winding of the electric motor and a partial discharge signal representing the partial discharge generated in the electric motor to which the impulse voltage is applied. As illustrated in FIG. 12, in the all-period partial discharge measurement method, one period includes a pulse supply period and a pulse pause period following the pulse supply period. In the first period, ten impulse voltages are applied during the pulse supply period (e.g., 20 ms) and application of the impulse voltage is stopped during the pulse pause period (e.g., 100 ms). Subsequently, in the second period, ten 10 V boosted impulse voltage are applied during the pulse supply period, and application of the impulse voltage is stopped during the pulse pause period. Such processing is repeated for all periods until a starting value (e.g., 1 kV) of a peak value of the impulse voltage reaches a stop value (e.g., 2 kV).
In the all-period partial discharge measurement method, in all the periods, an applied voltage signal representing the impulse voltage and the above-mentioned partial discharge signal are stored in a memory. Thereafter, an operator verifies, on a per-period basis, whether or not the partial discharge frequency is a specified frequency or more (e.g., five or more) and defines, as the partial discharge starting voltage, the impulse voltage applied to the twist pair sample in a period in which the partial discharge frequency first reaches a specified frequency or more. That is, in the all-period partial discharge measurement method, application of the impulse voltage and measurement of the partial discharge are continuously performed for all periods from the start of the application of the impulse voltage to stop thereof, and the partial discharge starting voltage is derived after completion of the measurement.
Further, in Non-Patent Document 2, a voltage increment of the impulse voltage to be boosted every period is set to be 10 V. Further, in one period, an interval between one impulse voltage and the subsequent impulse voltage in the pulse supply period is set to be 20 ms, and the pulse pause period is set to be 100 ms. The above settings are based on the specification of an impulse voltage generator to be used and cannot be set arbitrarily by a user.